Wafer carrier for batch wafer polishing in wafer polishing machines

ABSTRACT

A carrier assembly includes carrier protrusions extending outward from a base plate front face, one protrusion for each wafer to be polished. Each carrier protrusion has a front face separated from the base plate front face. An optional detachable ring may surround each protrusion front face to form a cavity for holding a wafer. In some examples of a carrier assembly, a frame having a separate aperture for each carrier protrusion holds all the detachable rings against the base plate front face. In other examples of a carrier assembly, the detachable rings are omitted, or a separate retaining ring for each carrier protrusion replaces the frame. A carrier protrusion may optionally be formed with edge relief to allow part of the wafer to deflect away from a polishing pad during polishing. A compressible elastic material may optionally be placed between a wafer and a carrier protrusion.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 61/646,761, filed May 14, 2012, incorporated herein by reference inits entirety.

FIELD OF THE INVENTION

Embodiments of the invention are related generally to carriers used inpolishing machines for polishing thin, flat work pieces and morespecifically to a wafer carrier for simultaneously polishing a batch ofwafers.

BACKGROUND

Thin, flat wafers may be used in the manufacture of semiconductordevices, optical components, and other products and applications. Awafer polishing machine may be used to remove material from at least onelarge face of a wafer to produce a polished wafer. A wafer polishingmachine may simultaneously polish a batch comprising more than one waferuntil all wafers in the batch meet specifications for polishing quality.Polishing quality may be represented by one or more parameters, forexample but not limited to variations in wafer thickness at differentlocations on a wafer, surface roughness, variations in thickness fromone wafer to another in a batch, amount of material removed from eachwafer in a batch, or other measured or calculated values.

During polishing, wafers may be held in a rotating carrier in a waferpolishing machine. A carrier may include a base plate having a frontside made uniformly flat and smooth by lapping. The back side of a baseplate may include mounting features for attaching the base plate to apolishing head in a polishing machine. Base plates may be made fromstiff materials such as stainless steel, aluminum alloy, or ceramic.Because wafers may be quite thin relative to their diameter, variationsfrom planarity of the base plate's front side may affect smoothness,flatness, or other parameters related to quality of polished wafers.Nearly the entire front side of a baseplate may be lapped smooth andflat, even when wafers being polished on the baseplate cover only afraction of the baseplate's surface area. Lapping a large area takesmore time and is more expensive than lapping a small area to the sameconditions of planarity and surface roughness.

A wafer to be polished may be placed in an aperture formed in a templatewith the front face of the wafer protruding outward from the frontsurface of the template. A template adapted for simultaneous polishingof more than one wafer may have a separate aperture for each wafer. Atemplate may include an upper layer made from fiberglass, anintermediate layer made from backing material, and a bottom layercomprising adhesive for attaching the template to the front face of thebase plate on the carrier. The intermediate layer in a template may bereferred to as the backing layer and the upper layer may be referred toas the front layer. During polishing, wafers are placed in aperturesformed in the upper and backing layers. Examples of wafer templatesattached to a lapped or polished front side of a base plate on a carrierare described in U.S. Pat. Nos. 3,449,870 to Jensen, 4,512,113 toBudinger, 5,193,316 to Olmstead, 5,267,418 to Currie et al., and6,612,905 to Nguyen. An example of a base plate and a cast aluminumalloy carrier is described in U.S. Pat. No. 4,194,324 to Bonora et al.

During polishing, wafers press against the edges of the apertures in thefront layer of a template, subjecting the template to wear along theedges of the apertures. Even a small amount of wear of the apertureedges may reduce the quality of polished wafers. Worn templates may bereplaced periodically, for example after a selected number of waferbatches have been polished. Removing a template from a carrier may bedifficult when adhesive has been used to bond the template to thecarrier. After a template is removed from a baseplate, residual adhesiveremaining on the baseplate may be removed by thorough cleaning toprovide a smooth, flat surface for installation of a new template. Thelapped front surface of a base plate may be damaged during removal of atemplate or cleaning of the base plate and may therefore be lapped againto restore the base plate to specifications for surface planarity andsurface roughness. A worn template may be discarded after being removedfrom a baseplate. Template replacement, baseplate cleaning, andbaseplate lapping represent significant contributors to wafer polishingcost.

The thicker a template's front layer, the better the template may resistdeformation and wear during polishing and the better it may able to holda wafer for achieving high quality polishing results. The front layer ofa template is preferably thinner than the thickness of a wafer to bepolished. The thinner the front layer, the higher the pressure that isapplied to edges of the front layer, increasing deformation and wear ofthe template edges that come into contact with wafers. Thesecontradictory requirements for template thickness may be difficult toresolve for thin wafers such as silicon wafers for semiconductorproduction, which may have a wafer thickness that is a small fraction ofwafer diameter.

The depth by which an aperture for holding a wafer penetrates into atemplate limits a range of wafer thicknesses which may be polished withthe template. The depth of an aperture may correspond to the thicknessof a template's front layer, or the depth may be made less than thethickness of the front layer by placing inserts in the aperture or byother means. A range of wafer thicknesses that may be polished in anyone template may be relatively narrow. A wafer that is too thick for thetemplate in which it is placed may escape from the template duringpolishing, possibly damaging the wafer or the template. If a wafer istoo thin for a particular template, the front surface of the wafer maynot protrude sufficiently outside the template to achieve desiredpolishing quality.

Variations in diameter from one wafer to another may prevent some wafersfrom being held securely in a template. Some wafers may be too large tofit within an aperture in a template. Other wafers may have a diameterthat is too small for a template to hold the wafers securely, allowingthe wafer to move excessively within the aperture. Excessive movement ofa wafer within a template aperture may reduce polishing quality. Fortemplates having more than one backing layer behind the front layer,dimensional tolerances for each layer add uncertainty to the preciseposition of a wafer relative to the base plate and may increaseuncertainty in the polishing results achievable for some combinations ofwafer thickness and polishing machine operating parameters.

A backing layer in a template may provide a planar surface against whichwafers are held during polishing. All wafers in a batch being polishedsimultaneously on one template may be placed against a same planarsurface defined by a backing layer in a template. Parts of a templatethat are behind a wafer, that is, between the back surface of a waferand the front surface of the base plate, may be compressed more thanother parts of the template by pressure applied to the wafer andtemplate during wafer polishing. A gap between the edges of a waferbeing polished and the side walls of the aperture in which the wafer isheld may permit excessive motion of the wafer relative to the templateand may lead to wear of the backing layer. One or more template layersmay wear or compress unevenly or at a different rate than other layers.One or more of these effects may lead to a template that becomesprogressively thinner or less flat, possibly causing polishing qualityto degrade over time and limiting the service lifetime of a template.Wafers polished on worn templates may not meet specifications forsurface planarity, surface roughness, or other specifications related topolishing quality. Template wear and compression may lead to a reductionin the yield of wafers that meet specifications for polishing quality.

Variations in backing layer tolerances, wear, compression, and otherfactors may cause uneven distribution of polishing slurry across theface of a wafer. Uniform distribution of polishing slurry across thefaces of wafers being polished may contribute to high quality polishingresults. Slurry deficiency may be especially pronounced near the centerof a wafer during polishing and may lead to inefficient or unevenremoval of material, possibly reducing the quality of polished wafers.

With some polishing machines, there may be an increasing deviation insurface flatness from the center of a polished wafer toward theperiphery. Wafer thickness near the edges of a wafer, sometimes referredto as “edge exclusion”, may decrease from the center of the wafer towardthe periphery of the wafer and may be referred to as “roll off”.Although less common, in some wafer polishing machines wafer thicknessincreases from the center of the wafer toward the wafer's periphery.Variations in wafer thickness may reduce the yield of wafers havingacceptable polishing quality.

SUMMARY

A carrier assembly for wafer polishing includes a base plate with a baseplate front face and at least three carrier protrusions extendingoutward from the base plate front face. Each carrier protrusion has aprotrusion front face separated from the base plate front face and fromany other of the at least three carrier protrusions. Each carrierprotrusion has a peripheral wall extending outward from the base plateto a perimeter of the protrusion front face. In some embodiments of theinvention, a frame is removably attachable to the base plate front face,for example with a plurality of threaded fasteners. The frame is formedwith a plurality of separate carrier protrusion apertures, one carrierprotrusion aperture for each carrier protrusion. A carrier assemblyembodiment of the invention may optionally further include at leastthree detachable rings. Each detachable ring is formed with a flange.The frame engages the flange on each detachable ring to hold the ringsagainst the base plate front face, with each detachable ring held withina separate carrier protrusion aperture around a different one of the atleast three carrier protrusions.

An alternative embodiment of a carrier assembly includes at least threecarrier protrusions, each having a carrier protrusion front face. Thecarrier assembly further includes at least three detachable rings, eachsurrounding a separate one of the at least three carrier protrusionswith a close fit, at least three retaining rings, each surrounding aseparate one of the at least three detachable rings with a close fit,and a base plate including a base plate front face and at least threering grooves formed in the base plate front face. Each of the at leastthree ring grooves includes a flat bottom surface parallel to the frontface on each of the at least three carrier protrusions. Each of the atleast three ring grooves surrounds a separate one of the at least threecarrier protrusions, thereby separating the baseplate front face fromthe carrier protrusion front face on each of the at least three carrierprotrusions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded pictorial view of an example of a carrier assemblyadapted for simultaneously polishing a batch of five wafers.

FIG. 2 is a view toward the front side of the example of a carrierassembly of FIG. 1.

FIG. 3 is a view toward a side of the example of a carrier assembly ofFIGS. 1-2.

FIG. 4 is a pictorial view of an example of a base plate with fivecarrier protrusions extending outward from the front side of the baseplate.

FIG. 5 is a view toward the front side of an example of a frame adaptedfor holding a batch of five wafers.

FIG. 6 is a pictorial view of an example of a detachable ring having aperipheral flange.

FIG. 7A is a cross-sectional view A-A of examples of a carrierprotrusion, a detachable ring, and a frame. The location of section A-Ais marked by section line A-A in FIG. 2.

FIG. 7B is an alternate view of section A-A, showing examples of a waferand an optional insert in a cavity formed by the detachable ring andfront face of the carrier protrusion of FIG. 7A.

FIG. 8 is a view toward the front side of an example of an alternativeembodiment of a carrier assembly having a frame with a separate aperturefor each carrier protrusion without a detachable ring between the frameand each carrier protrusion.

FIG. 9 is a cross-sectional view E-E of the examples of a carrierprotrusion and a frame from the carrier assembly of FIG. 8, showing thatthe frame does not contact the carrier protrusion in the illustratedexample. The location of section E-E is marked by section line E-E inFIG. 8.

FIG. 10 is a view toward the front surface of an example of a frame on acarrier assembly adapted for polishing a batch of three waferssimultaneously.

FIG. 11 is an exploded pictorial view of the example of a carrierassembly of FIG. 10, showing a front side of a base plate with threecarrier protrusions, three detachable rings, and a frame with threecarrier protrusion apertures.

FIG. 12 is a view toward the front side of an example of alternativeembodiment of the invention comprising a base plate with three carrierprotrusions, a detachable ring surrounding each carrier protrusion, andthree retaining rings for holding the detachable rings against the baseplate.

FIG. 13 is an exploded pictorial view of the example of a carrierassembly of FIG. 12, showing examples of grooves formed in the frontsurface of the base plate and examples of retaining rings for holding adetachable ring in each groove.

FIG. 14A is a cross-sectional view B-B of examples of a carrierprotrusion, a detachable ring surrounding the carrier protrusion, and aretaining ring surrounding the detachable ring and holding thedetachable ring to the base plate. The location of section B-B is markedby section line B-B in FIG. 12.

FIG. 14B is an alternate view of section B-B, showing examples of awafer and an optional insert in a cavity formed by the detachable ringand the front face of the carrier protrusion from FIG. 14A.

FIG. 15 is a cross-sectional view C-C of an example of a carrierprotrusion with edge relief in the form of a chamfer all the way aroundthe periphery of the front face of a carrier protrusion, and furthershowing examples of a wafer and an optional insert in their undeflectedpositions before polishing begins. The location of section C-C is markedby section line C-C in FIG. 12.

FIG. 16 is a cross-sectional view C-C of an alternative example of acarrier protrusion with edge relief formed as a step all the way aroundthe front peripheral edge of a carrier protrusion.

FIG. 17 is an alternative cross-sectional view C-C of the example of acarrier protrusion of FIG. 16, showing an example of an elastic materialplaced between the back side of the wafer and base plate in theperipheral step around a carrier protrusion.

FIG. 18 is a view toward the front side of a carrier protrusion withedge relief as shown in the example of FIG. 17, and an example of anelastic ring positioned on the peripheral step.

FIG. 19 is another alternative cross-sectional view C-C of the exampleof a carrier protrusion of FIG. 16, showing an example of a carrierprotrusion formed with a front face having a curved profile.

FIG. 20 is a cross-sectional view of an example of a carrier assemblywith a frame and a carrier protrusion with edge relief in the form of aperipheral step. The location of section D-D is marked by section lineD-D in FIG. 10.

DESCRIPTION

Embodiments of the invention comprise a wafer carrier for simultaneouslypolishing a batch of wafers on a wafer polishing machine. As usedherein, a batch refers to all the wafers being polished simultaneouslyon one wafer carrier embodiment of the invention. An embodiment of theinvention includes a base plate having at least three carrierprotrusions on the front side of the base plate. Each carrier protrusionhas a front face for supporting one wafer. The carrier protrusion frontface against which a wafer is held during polishing is separated fromthe carrier protrusion front face against which any other wafer is held.In some embodiments of the invention, a carrier protrusion front face isformed as a flat, smooth surface. In alternative embodiments of theinvention, the front face of a carrier protrusion may be formed with acurved, chamfered, or stepped profile so that the height of a carrierprotrusion is less near the perimeter of the carrier protrusion thannear the center. A carrier protrusion with a curved, chamfered, orstepped profile may have advantages for controlling edge exclusion orroll off effects compared to carriers with flat support surfaces forwafers.

In contrast to embodiments of the invention, carriers previously knownin the art have a base plate with a front face that is made flateverywhere across the front face. For carriers previously known in theart, all the wafers in one batch are supported by the single,approximately planar surface of the backing layer of a template attachedto the base plate's front face, rather than resting on separatedsurfaces as in an embodiment of the invention.

The examples of a wafer carrier described herein have a front face witha circular perimeter shape, although other shapes may be used. The frontface of the wafer carrier is the face immediately adjacent to apolishing pad during wafer polishing in a wafer polishing machine. Ingeneral, the front side or face of components described herein will bethe side facing the polishing pad during wafer polishing. For example,the front face of a wafer refers to the face being polished by contactwith the polishing pad. The back face of a wafer faces toward the frontface of a carrier protrusion during wafer polishing. One or more insertsor spacers may optionally be interposed between the back face of a waferand the front face of a carrier protrusion. A carrier, also referred toherein as a carrier assembly or wafer carrier, may be driven in rotationduring wafer polishing by a drive shaft or other components coupled tothe back side of the carrier. Components for driving a carrier inrotation may optionally be omitted from an embodiment of the invention.

A reference to a thickness dimension of an element, for example a waferthickness, a thickness of a carrier protrusion, or a thickness of aframe, refers to a direction normal to a front face on a base plate whenthe element is in position for wafer polishing. A dimension or directionreferred to as thickness may alternately be referred to as height.

A workpiece that may be processed by an embodiment of the invention isreferred to herein as a wafer. A wafer suitable for use with anembodiment of the invention comprises at least one relatively large faceto be polished to specified conditions of planarity and smoothness,possibly expressed by values related to surface roughness, dimensions ofpolishing marks, deviation from an ideal surface, or other valuesrelated to polishing quality. A wafer may alternatively have, forexample but not limited to, a circular perimeter, a polygonal perimeter,a curved perimeter with at least one straight edge, or an irregularperimeter shape. A silicon wafer used in the manufacture of integratedcircuits is an example of a workpiece referred to herein as a wafer, butother relatively thin, relatively flat workpieces may also be suitablefor processing by embodiments of the invention. Embodiments of theinvention are especially well suited for polishing wafers that are verythin compared to their largest linear dimension. For some applications,polished wafers preferably have uniform thickness and very low surfaceroughness on at least one polished face.

Referring now to the figures, an exploded view of an example of a wafercarrier embodiment of the invention is shown in FIG. 1. The example of awafer carrier 10, also referred to as a carrier assembly 10, includes abase plate 12 adapted for attachment to a drive shaft in a waferpolishing machine (not illustrated) for driving the carrier assembly 10in rotation during wafer polishing. The example of a carrier assembly 10in FIG. 1 shows an example of a base plate 12 having five carrierprotrusions 14 integrally formed onto the front side of the base plate.In some embodiments of a base plate 12, each carrier protrusion 14extends outward from a base plate front face 18. Each carrier protrusion14 includes a protrusion front face 16 that is spatially separated fromthe base plate front face 18 and from the front face 16 of any othercarrier protrusion 14. For example, in some embodiments of the inventiona protrusion front face 16 may be positioned outward from the base platefront face 18 by a thickness dimension of a carrier protrusion 14. Acarrier protrusion 14 may be attached to or alternately formed as anintegral part of the base plate front face 18.

A separate carrier protrusion 14 is provided for each wafer 60 to bepolished on the carrier assembly 10. Each of the five wafers 60 shown inbroken lines in FIG. 1 is an example of a work piece having a frontsurface 62 to be polished by contact with polishing slurry and arotating polishing pad (not illustrated). The front surface 62 of awafer 60 may be referred to as the front face of the wafer. An optionalinsert 50, also shown in broken lines, may be placed between a backsurface of each wafer 60 and the front face 16 of each carrierprotrusion 14. When an insert is omitted, the back surface of a wafer 60may contact the front face 16 of a carrier protrusion 14 duringpolishing of the front surface 62 of the wafer 60.

A detachable ring 40 may optionally be provided for each carrierprotrusion 14. A detachable ring 40 may optionally be made from aplastic material and may optionally fit around a carrier protrusion witha close fit. In the carrier assembly example of FIG. 1, detachable rings40 are interposed between the sides of the carrier protrusions 14 andthe edges of apertures 32 in a frame 30 and are retained against thebase plate front face 18 by the frame 30. The frame 30, also referred toas a mounting plate, is formed with one carrier protrusion aperture 32for each carrier protrusion 14 on the base plate 12. The frame may beformed with countersunk through-holes 22 through which threadedfasteners (not shown in FIG. 1) pass to removably assemble the frame 30to the base plate 12. When the frame 30 is assembled to the base plate12, the detachable ring 40 surrounding each carrier protrusion 14 fitswithin a corresponding carrier protrusion aperture 32. The detachablerings 40 prevent contact between wafers 60 and the frame 30. Duringwafer polishing, the front surface 62 of each wafer 60 extends outwardfrom the front surface 64 of the frame 30 so that the front surface ofthe wafer may come into contact with a polishing pad and polishingslurry in a wafer polishing machine. Each carrier protrusion, protrusionfront face, detachable ring, and protrusion aperture in the frame mayoptionally have a perimeter shape corresponding to the perimeter shapeof a wafer to be polished. Alternatively, a carrier protrusion,detachable ring, aperture in a frame, or other parts of an embodiment ofthe invention may have a perimeter shape that matches part of theperimeter of a wafer to be polished but does not match other parts ofthe wafer's perimeter. A wafer may be held firmly enough by thedetachable ring 40 and frame 30 to prevent the wafer 60 from escapingfrom the detachable ring during wafer polishing.

A wafer 60 may slide laterally relative to the front face 16 of thecarrier protrusion 14 behind the wafer, possibly resulting in wear ofthe detachable ring 40. For frames adapted for use with detachablerings, the detachable rings 40 reduce wear of the frame 30 and may bereplaced more frequently and at lower cost than the frame 30, extendingthe service lifetime of the frame and base plate. A frame 30 adapted foruse with detachable rings 40 may have a substantially longer servicelife than a template used with polishing machines previously known inthe art. A carrier assembly 10 having a base plate 12 with carrierprotrusions 14, a frame 30, and detachable rings 40 may have asubstantially longer service lifetime before re-lapping is required thancarrier assemblies previously known in the art.

The example of a carrier assembly 10 from FIG. 1 is shown in a viewtoward the front surface 64 of the frame 30 in FIG. 2 and in a side viewin FIG. 3. The example of a frame 30 in FIG. 2 is formed with five (5)carrier protrusion apertures 32. A detachable ring 40 surrounds eachcarrier protrusion 14 and fits within the corresponding carrierprotrusion aperture 32. The frame 30 may be made of a stiffer materialthan the rings 40 to reduce flexure of the rings and to reduce wafermotion relative to the carrier assembly 10 during wafer polishing. Aflange 42 having an outer flange radius 86 from the center of thedetachable ring extends around the back side of each detachable ring 40and fits in a channel formed on the back side of the frame 30 aroundeach carrier protrusion aperture 32, holding the detachable rings 40between the frame 30 and the front surface of the base plate 12 in thecarrier assembly 10. In the example of FIG. 2, the cylindrical carrierprotrusions 14 have a circular perimeter shape with a radius 82corresponding to an inner radius of a detachable ring 40. The carrierprotrusion apertures 32 are formed with a radius 84, corresponding to anouter radius of a detachable ring 40. The centers of the cylindricalcarrier protrusions 14 are located on a circle having a radius 88. Thecenter of each carrier protrusion 14 is separated from the centers ofadjacent carrier protrusions on either side by equal angular intervals90. A plurality of threaded fasteners 70 pass through countersunkapertures in the frame 30 and engage corresponding threaded apertures inthe base plate 12 to enable removable attachment of the frame 30 to thebase plate 12. The front face 16 of a carrier protrusion is visible inFIG. 2 within the area surrounded by a detachable ring 40.

FIG. 4 shows a pictorial view of an example of a base plate 12 adaptedfor simultaneous polishing of a batch of five wafers. Five carrierprotrusions 14 extend outward from the base plate front face 18. Thefront face 16 of each carrier protrusion 14 is joined to the base platefront face 18 by a peripheral wall 20 forming the side of each carrierprotrusion. The height of the peripheral wall 20 corresponds to athickness of a carrier protrusion at the periphery of the carrierprotrusion. In the example of FIG. 4, the peripheral wall 20 spatiallyseparates a carrier protrusion front face 16 from the base plate frontface 18.

A view toward the front surface 64 of an example of a frame 30 is shownin FIG. 5. The frame 30 is formed with five carrier protrusion aperturesspaced at equal angular intervals, one aperture for each carrierprotrusion on the base plate. The frame 30 may also be formed withcountersunk through-holes 22 through which threaded fasteners pass toassemble the frame to a base plate. Each carrier protrusion aperture 32has a peripheral edge 38 which may contact and support an optionaldetachable ring around each carrier protrusion. As shown in the exampleof FIG. 5, an “I”-shaped aperture 36 may optionally be formed in thefront surface 64 of the carrier 30 in each of the places where adjacentcarrier protrusion apertures 32 are closest to one another. The length,width, and depth dimensions of an “I”-shaped aperture may be selected toprovide a desired flow rate of polishing compound through the gapbetween adjacent wafers during wafer polishing. The “I”-shaped aperturesare effective for maintaining a uniform distribution of polishingcompound across the face of each wafer. The apertures 36 mayalternatively be formed as straight channels or other shapes. In a frame30 lacking “I”-shaped apertures 36, polishing compound may not bedistributed evenly to all areas on the front surfaces of wafers beingpolished. “I”-shaped apertures 36 may optionally penetrate the frontlayer of the frame 30 without penetrating any of the frame's backinglayers, or may alternatively penetrate the front layer and a selectednumber of other layers.

A pictorial view toward the front side of an example of a circulardetachable ring 40 is shown in FIG. 6. An inner wall 43 of thedetachable ring example of FIG. 6 is optionally sized for a close fitaround the peripheral wall of a carrier protrusion. The thickness orheight of the detachable ring may optionally be somewhat greater thanthe thickness of a frame into which the detachable ring fits. Theperipheral shape of a detachable ring, represented by the shape of aninner edge 38, is approximately the same as the peripheral shape of acarrier protrusion and of a wafer to be polished. The outer wall 45 ofthe detachable ring 40 may optionally be sized for a close fit into acarrier protrusion aperture in a frame. A flange 42 extends around theperiphery of the back side of the outer wall 45. The flange 42 fits intoa corresponding channel formed into the back side of the frame aroundeach carrier protrusion aperture. The back side of the detachable ring40 (back side not visible in FIG. 6) contacts a front surface of thebase plate in a carrier assembly. A detachable ring 40 may optionally beformed with a split 44 through the outer wall 45 and flange 42, and mayoptionally include at least one transverse groove 46 through whichpolishing compound may flow during wafer polishing, thereby providinguniform distribution of polishing compound across the front face of awafer being polished and improving the quality of polished wafers.

The front face of a carrier protrusion and the inner wall of adetachable ring form a cavity into which a wafer fits for waferpolishing, with the front surface of the wafer extending outward fromthe front surface of the frame. For embodiments of the invention havinga frame, the front surface of the frame is farther from the base platefront face than the protrusion front face. The front face of a carrierprotrusion may be formed by conventional metalworking processes, forexample by lapping, and may be referred to as the lapped face of acarrier protrusion. An example of a cavity for holding a wafer is shownin FIGS. 7A and 7B. FIG. 7A represents a view of cross section A-A fromthe example of FIG. 2. FIG. 7B is an alternative view of cross sectionA-A having a wafer 60 and an optional insert in the cavity formed by thedetachable ring and carrier protrusion, with the wafer front surface 62extending above the detachable ring 40 and front surface 64 of the frame30.

FIG. 7A shows an example of a cross-section A-A through the base plate12, detachable ring 40, and frame 30 of FIG. 2. Parts of a carrierassembly behind the base plate 12 (ref. FIG. 3) are omitted in FIGS.7A-7B and other cross-sectional views. The carrier protrusion 14 in theexample of FIG. 7A may optionally be formed as an integral part of thebase plate 12. In alternative embodiments of the invention, carrierprotrusions may be formed separately from the base plate and thenattached to the base plate. The protrusion front face 16 is separatedfrom the base plate front face 18 by a peripheral wall 20 around theperimeter of the carrier protrusion 14. Unlike carriers previously knownin the art, the front face 18 of the base plate 12 may not be lapped.The front face 18 of the base plate 12 may therefore be less smooth thanthe front face 16 of a carrier protrusion. The detachable ring 40 fitsagainst the peripheral wall 20 of the carrier protrusion 14, extendingoutward from the protrusion front face 16 and outward from the front ofthe frame 30. The frame 30 traps the flange 42 on the detachable ring 40against the front face 18 of the base plate 12. Threaded fasteners (notshown in the examples of FIGS. 7A-7B) may be used to attach the frame 30to the base plate 12.

Alternative Section A-A in FIG. 7B repeats the example of FIG. 7A andadds an example of a wafer 60 to be polished and an example of anoptional insert 50. The front face 62 of the wafer 60 protrudes beyondthe front side of the detachable ring 40 and the frame 30 so thatmaterial may be removed efficiently and evenly from the front faceduring wafer polishing. As suggested in the example of FIG. 7B, thelargest dimension of the wafer is smaller than the inner diameter of thedetachable ring 40. Lateral motion of the wafer 60 relative to the frontsurface 16 of the carrier protrusion 14 may cause frictional wear of thedetachable ring 40, but the detachable ring prevents frictional wear ofthe frame 30 by the wafer 60.

Some embodiments of a frame have carrier protrusion apertures into whichcarrier protrusions are positioned without an intervening detachablering. An example of a frame 30 adapted for polishing wafers without theuse of detachable rings is shown in FIG. 8 and FIG. 9. As suggested inFIG. 8, the frame 30 is formed with carrier protrusion apertures 32 intowhich carrier protrusions 14 fit so that the front face 16 of eachcarrier protrusion 14 is located within an aperture 32. The inner wall39 of each aperture 32 is preferably separated from the peripheral wall20 of each carrier protrusion 14 so that the inner wall 39 does notcontact the peripheral wall 20. The frame 30 along the front surface 64is thicker than the height of a carrier protrusion 14 so that a cavityfor holding a wafer to be polished is formed by the inner wall 39 of thecarrier protrusion aperture 32 in the frame 30 and the front face 16 ofthe carrier protrusion 14. In an embodiment of the invention whichoperates without detachable rings, for example as shown in FIGS. 8-9,the edges of a wafer being polished may come into direct contact withthe inner wall 39 of a carrier protrusion aperture 32 because there isno detachable ring intervening between the wafer and frame 30. Althoughthe example of a frame 30 in FIG. 8 is adapted for polishing fivewafers, in alternative embodiments of the invention the frame is adaptedfor polishing three wafers or other numbers of wafers.

An example of an alternative embodiment of the invention adapted forsimultaneous polishing of a batch of three wafers is shown in FIG. 10.The example of a carrier assembly 10 of FIG. 10 includes a frame 30having a front surface 64. The frame 30 may be removably attached to abase plate 12 by threaded fasteners 70. Three carrier protrusionapertures 32 formed in the frame 30 are spaced at equal angularintervals 90 around the frame. A detachable ring 40 fits within eachcarrier protrusion aperture 32 and surrounds a front face 16 on each ofthree carrier protrusions 14. In the example of FIG. 10, each carrierprotrusion front face 16 has a circular perimeter shape with a radius 82corresponding to an inner radius of a detachable ring 40. Eachdetachable ring 40 fits around and against the sides of a separatecarrier protrusion 14. A peripheral flange 42 having an outer radius 86on each detachable ring 40 contacts the back side of the frame 30,enabling the frame 30 to hold the detachable rings 40 against the baseplate 12. The center of each carrier protrusion 14 is positioned on acircle having a radius 88 originating at a rotational center of the baseplate. An outer diameter 84 of a detachable ring 40 corresponds to adiameter of a carrier protrusion aperture 32.

The example of a carrier assembly 10 with a frame 30, three flangeddetachable rings 40, and a baseplate 12 with three raised carrierprotrusions 14 is shown again in the pictorial view of FIG. 11. FIG. 11further illustrates an example of a threaded fastener 70 which passesthrough a countersunk through-hole 22 in the frame 30 and engages acorresponding threaded hole 34 formed in the front face 18 of the baseplate 12 for retaining the frame and rings against the base plate. Acarrier assembly 10 may optionally use a different number and locationof threaded fasteners 70, countersunk holes 22, and threaded holes 34than are shown in the example of FIG. 11. Dashed lines in FIG. 11 showthe intervening position of a detachable ring 40 between a carrierprotrusion 14 and the frame 30 and the alignment of the front face 16 ofa carrier protrusion and detachable ring 40 with a carrier protrusionaperture 32 in the frame.

In the examples of a carrier assembly from FIGS. 1 and 9, carrierprotrusions extend outward from the front face of a base plate, adetachable ring fits around each carrier protrusion, and each detachablering and carrier protrusion fit into a corresponding aperture in aframe. In an alternative embodiment of a carrier assembly, the frontface of each carrier protrusion may be isolated from the front face ofthe base plate by a groove around the perimeter of each carrierprotrusion. However, instead of a frame for holding more than onedetachable ring to the base plate as in the examples of FIG. 1 and FIG.11, a separate retaining ring may be provided for individually holdingeach detachable ring to the baseplate. A retaining ring functions likethe frame in other embodiments of the invention for supporting adetachable ring against a wafer protrusion. However, each retaining ringmay be attached to or removed from a base plate independently from otherretaining rings on the same baseplate.

FIG. 12 shows a view toward the front face 18 of an example of a baseplate 12 adapted for holding a detachable ring 40 in place around acarrier protrusion front face 16 with a retaining ring 48. The carrierassembly 10 of FIG. 12 is configured for simultaneously polishing abatch of three wafers. In the example of FIG. 12, the carrierprotrusions 14 have a circular perimeter shape with a radius 82corresponding to an inner radius of a detachable ring 40. The detachablering 40 has an outer radius 84 corresponding to an inner radius of theretaining ring 48. The detachable ring 40 has a circumferential flangewith an outside radius 86 that fits in a corresponding step formed inthe back side of the retaining ring 48. The centers of the circularperimeters of the carrier protrusions 14 are positioned on a circle ofradius 88 from the rotational center of the base plate 12. An outerradius 92 of the retaining ring 48 is chosen so that the three retainingrings, carrier protrusions 14, and detachable rings 40 may be spaced atequal angular intervals 90. In FIG. 12, the front face 16 of eachcarrier protrusion 14 is visible within the area surrounded by eachdetachable ring 40.

FIG. 13 shows an exploded pictorial view of the example of a carrierassembly 10 from FIG. 12. Dashed lines indicate how a flange 42 on eachdetachable ring 40 may be held by a retaining ring 48 in a ring groove54 formed in the front face 18 of the base plate 12. The front face 16of each carrier protrusion 14 is spatially separated from the front face18 of the base plate 12 and from the front faces of other carrierprotrusions by the ring groove 54. Retaining rings 48 may be removablyattached to the base plate 12 by threaded fasteners (not illustrated)passing through apertures 22 in the retaining rings and engagingthreaded holes 34 in the flat bottom surface 56 of each ring groove 54.Retaining rings 48 are preferably fabricated from a strong, chemicallyinert material, for example stainless steel.

FIG. 14A shows an example of a cross-section B-B through the base plate12, detachable ring 40, and retaining ring 48 of FIG. 12. The carrierprotrusion 14 in the example of FIG. 14A may be formed by a groove 54 inthe base plate front face 18. The groove 54 defines the outer perimetershape of the carrier protrusion, with an inner wall of the groovecorresponding to the outer peripheral wall 20 of the carrier protrusion.The groove spatially isolates the front face of a carrier protrusionfrom other front-facing surfaces on a base plate. The perimeter shapesof carrier protrusions, retaining rings, and detachable rings may matchthe perimeter shapes of wafers to be polished.

The bottom surface 56 of the groove 54 may be parallel to the front face16 of a carrier protrusion 14. The groove 54 may optionally be wideenough for the retaining ring 48 and detachable ring 40 to fit into thegroove. Alternatively, the groove may be wider than the combined widthsof the retaining ring and ring. The thickness of the base plate 12 inthe region of the carrier protrusion 14 may be approximately the same asthe thickness of the base plate just outside the area enclosed by aretaining ring 48 on the front face 18 of the base plate 12. In analternative embodiment of the base plate 12, the thickness of the baseplate in the region of the carrier protrusion is less than the thicknessof the base plate outside the retaining ring, that is, the front surface16 of the carrier protrusion 14 is below the front surface 18 of thebase plate in the orientation shown in FIG. 14A. In another alternativeembodiment of the invention, the thickness of the base plate in theregion of the carrier protrusion is greater than the thickness of thebase plate outside the retaining ring, that is, the front surface 16 ofthe carrier protrusion 14 is above the front surface 18 of the baseplate in the example of FIG. 14A.

FIGS. 14A and 14B show an example of a detachable ring 40 held in thegroove 54 by the retaining ring 48. An inner wall 43 of the detachablering 40 surrounds the outer peripheral wall 20 of the carrier protrusion14 with a close fit. The detachable ring 40 includes a peripheral flange42 that engages a corresponding overhang or step 47 formed on the backside of the retaining ring 48. A back side of the flange 42 contacts thebottom surface 56 of the groove 54 in the base plate 12. The outer wall45 of the detachable ring 40 fits against the corresponding inner wall49 of the retaining ring 48 with a close sliding fit. At least twothreaded fasteners (not illustrated) pass through countersunk holes 22in the retaining ring 48 and engage threaded holes 34 in the base plate12 to hold the retaining ring 48 and detachable ring 40 tightly againstthe bottom surface 56 of the groove 54. FIG. 14B further illustrates anexample of a wafer 60 to be polished, with an optional insert 50 placedbetween a back side of the wafer and the front face 16 of the carrierprotrusion 14. As suggested in FIG. 14B, a gap may exist between theedges of the wafer 60 and the inner wall 43 of the detachable ring 40.The front surface 62 of the wafer 60 preferably protrudes beyond thesteel ring 48 and detachable ring 40 during wafer polishing. A wafer maybe held firmly enough by the detachable ring 40 and retaining ring 48 toprevent the wafer 60 from escaping from the ring during wafer polishing.

In the examples of carrier protrusions shown in cross sectional views7A, 7B, 12A, and 12B, the front surface 16 of a carrier protrusion hasbeen drawn in profile as a straight line to represent a planar surface.In alternative embodiments of the invention, a carrier protrusion mayhave a surface with a curved profile, chamfered edges, stepped edges, orsome other deliberate deviation from planarity. A deviation fromplanarity in the front surface of a carrier protrusion permits part of awafer to deflect during wafer polishing, possibly causing differentrates of removal of material at different locations on a wafer'ssurface. A nonplanar profile for a wafer carrier front surface mayenable compensation during wafer polishing for non-uniform distributionof polishing compound, misalignment of various axes of rotation in awafer polishing machine, or other factors that influence the quality ofpolished wafers. Shaping the surface of a carrier protrusion with adeliberate deviation from planarity may reduce uncertainties related toplacing one or more layers of inserts between a wafer and carrierprotrusion and may therefore improve the yield and reduce the cost ofacceptably polished wafers.

An example of a carrier protrusion having a deliberate deviation fromplanarity, implemented as edge relief, is shown in the cross-sectionalview of FIG. 15. Section C-C in FIG. 15 shows a partial cross section ofthe base plate 12, retaining ring 48, and detachable ring 40 in a groove54 from the example of a carrier assembly 10 in FIG. 12. The detachablering 40 fits close to the outer peripheral wall 20 of a carrierprotrusion as previously explained with regard to FIGS. 12A-12B. Achamfer 94A is formed inward from the outer peripheral wall 20 along thefront face 16 of the carrier protrusion. The chamfer extends inward adistance L 78 from the outer peripheral wall 20 of the carrierprotrusion, all the way around the carrier protrusion. The thickness ofthe carrier protrusion at the outer peripheral wall 20 differs from thethickness inward of the chamfer 94A by a maximum dimension H 76. Theheight dimension H 76 represents a maximum amount of deflection of thewafer 60 toward the base plate 12 over a length D 74 from the side 66 ofthe wafer 60 during wafer polishing. Neither deflection of the wafer 60during polishing nor quality of the polished wafer surface 62 isaffected by planarity or smoothness of the front surface 18 of the baseplate 12.

FIG. 15 illustrates an example of a reference position for the wafer 60and optional spacer 50 corresponding to an undeflected position of thewafer, before the front surface 62 of the wafer is pressed into apolishing pad. During polishing, the back surface 80 of the wafer 60bends closer to the chamfer 94A, thereby reducing an amount of materialremoved from the wafer over the distance D 74 compared to materialremoved from other parts of the wafer. If an optional insert 50 ispresent, the insert bends closer to the chamfer when the wafer bends.The height of the chamfer H 76 and the length of the chamfer L 78 mayoptionally be selected to cause a selected local variation in the rateof material removal across the front face 62 of the wafer 60. Thechamfer 94A may optionally be formed with an acute angle between thechamfered face and a horizontal reference, for example the front surface16 of the carrier protrusion, in a range from 0.02 degrees to 5 degrees.

FIG. 16 shows an alternative embodiment of a carrier protrusion withedge relief. Instead of a chamfer as in the example of FIG. 15, thecarrier protrusion in the example of FIG. 16 is formed with a step 94Bcut inward from the outer peripheral wall 20 toward the center of thecarrier protrusion. As shown in alternative cross-sectional view C-C inFIG. 16, the step 94B is formed with width L 78 and depth H 76 all theway around the carrier protrusion. The back surface 80 of the wafer 60deflects toward the base plate 12 over a distance D 74 from the side 66of the wafer during wafer polishing. A magnitude of deflection may becontrolled by parameters related to the operation of the wafer polishingmachine, for example an amount of pressure applied to the carrierassembly to force the wafers being polished into a polishing pad,surface characteristics of an insert, surface characteristics of apolishing pad, properties of the polishing slurry, and possibly otherparameters.

A compressible, elastic material may optionally be placed in the step94B to affect an amount by which the wafer 60 deflects toward the baseplate 12 during wafer polishing. An example of an elastic material inthe form of an elastic ring 52 is shown in the alternate cross sectionC-C in FIG. 17. The elastic ring 52 is positioned on the step 94B on thecarrier protrusion and contacts the optional insert 50. When theoptional insert is omitted, the elastic ring contacts a back surface 80of the wafer 60. The carrier protrusion and elastic ring from theexample of FIG. 17 are shown in a view toward the front surface 16 ofthe carrier protrusion 14 in FIG. 18. The carrier protrusion 14 has aradius 82 from the center of the protrusion to the outer peripheral wall20. Edge relief 94 extends inward a distance L 78 from the outerperipheral wall 20 of the carrier protrusion 14. Edge relief 94 mayalternatively be implemented as a chamfer, a step, or a continuouscurved surface, with a thickness of the carrier protrusion at the outerperipheral wall 20 less than or equal to a thickness of the carrierprotrusion elsewhere on the front face 16. FIG. 19 shows across-sectional view of an example of a carrier protrusion having edgerelief implemented as a smoothly curved surface 94C from the frontsurface 16 outward to the peripheral wall 20. In the example of FIG. 19,the carrier protrusion has a greater height near the left side of thefigure than near the perimeter of the protrusion. FIG. 19 furtherillustrates an example of a wafer having a back surface 80 in directcontact with the front surface 16 of a carrier protrusion, without anintervening insert between the wafer and carrier protrusion.

In an embodiment of a carrier protrusion with edge relief, an optionalelastic ring 52, when present, rests on the carrier protrusion 14 in thearea of the edge relief 94. The elastic ring 52 may optionally beattached to the carrier protrusion 14 with adhesive. The elastic ring 52may optionally be formed with a perimeter shape that matches theperimeter shape of wafers to be polished.

FIG. 18 may be viewed as an example of a carrier protrusion with edgerelief in a carrier assembly having a frame. Section D-D in FIG. 20shows more details of a carrier protrusion with edge relief on a baseplate adapted for attachment of a frame. Section D-D shows a partialcross-sectional view of an alternative embodiment of the base plate 12from the example of FIG. 10 in which the detachable ring 40 is heldagainst the outer peripheral wall 20 of a carrier protrusion and againstthe front face 18 of the base plate 12 by a frame 30. As shown in FIG.20, a carrier protrusion may be formed with edge relief on the frontface 16 of the carrier protrusion. The edge relief may alternatively beimplemented as a step 94B as shown in FIG. 20 or as a chamfer or curvedsurface as described above for carrier assemblies without frames. A ringof elastic material 52 may optionally be interposed between the wafer 60and the base plate 12, or between the base plate 12 and an optionalinsert 50 if present. During wafer polishing, the wafer 60 and theoptional insert 50 deflect toward the base plate 12 in the region of theedge relief.

Unless expressly stated otherwise herein, ordinary terms have theircorresponding ordinary meanings within the respective contexts of theirpresentations, and ordinary terms of art have their correspondingregular meanings.

What is claimed is:
 1. A carrier assembly for wafer polishing,comprising: a base plate comprising a base plate front face; at leastthree carrier protrusions extending outward from said base plate frontface, wherein each of said carrier protrusions comprises: a protrusionfront face separated from said base plate front face and from any otherof said at least three carrier protrusions; and a peripheral wallextending from said base plate to a perimeter of said protrusion frontface; and a frame comprising a front surface, wherein said frame isremovably attachable to said base plate front face with a plurality ofthreaded fasteners, said frame is formed with a separate carrierprotrusion aperture for each of said at least three carrier protrusions,and each of said carrier protrusion extends into a separate one of saidcarrier protrusion aperture with said front surface of said framefarther from said base plate front face than each of said protrusionfront face.
 2. The carrier assembly of claim 1, further comprising atleast three detachable rings, wherein each of said at least threedetachable rings fits around a different one of said at least threecarrier protrusions.
 3. The carrier assembly of claim 2, wherein each ofsaid at least three detachable rings comprises a flange and said frameholds said flange against said base plate front face.
 4. The carrierassembly of claim 2, wherein each of said at least three detachablerings further comprises: an inner wall; an outer wall; and a pluralityof transverse grooves from said inner wall to said outer wall.
 5. Thecarrier assembly of claim 1, wherein said base plate consists of five ofsaid carrier protrusions.
 6. The carrier assembly of claim 1, whereinsaid protrusion front face on each of said at least three carrierprotrusions has a peripheral shape corresponding to a peripheral shapeof a wafer to be polished.
 7. The carrier assembly of claim 1, whereinsaid frame further comprises a front face, said front face is formedwith a separate carrier protrusion aperture for each of said at leastthree carrier protrusions, and said front face is formed with at leastthree grooves, one of said at least three grooves between each adjacentpair of said carrier protrusion apertures.
 8. The carrier assembly ofclaim 1, wherein each of said at least three carrier protrusions isformed with a chamfer from said peripheral wall to said protrusion frontface.
 9. The carrier assembly of claim 1, wherein each of said at leastthree carrier protrusions is formed with a step from said peripheralwall to said protrusion front face.
 10. The carrier assembly of claim 9,further comprising an elastic ring on said step formed on each of saidat least three carrier protrusions.
 11. A carrier assembly, comprising:at least three carrier protrusions, each of said at least three carrierprotrusions comprising a carrier protrusion front face; at least threedetachable rings, each of said at least three detachable ringssurrounding a separate one of said at least three carrier protrusions;at least three retaining rings, each of said at least three retainingrings surrounding a separate one of said at least three detachablerings; and a base plate comprising: a base plate front face; and atleast three ring grooves formed in said base plate front face, each ofsaid at least three ring grooves having a flat bottom surface parallelto said carrier protrusion front face on each of said at least threecarrier protrusions, and each of said at least three ring groovessurrounding a separate one of said at least three carrier protrusions,thereby separating said base plate front face from each of said carrierprotrusion front face.
 12. The carrier assembly of claim 11, whereineach of said at least three detachable rings comprises a flange and saidat least three retaining rings hold said flange for each of said atleast three detachable rings against said flat bottom surface in each ofsaid at least three ring grooves.
 13. The carrier assembly of claim 11,wherein said carrier protrusion face on each of said at least threecarrier protrusions has a perimeter shape corresponding to a perimetershape of a wafer to be polished.
 14. The carrier assembly of claim 11,consisting of: five of said at least three carrier protrusions; five ofsaid at least three detachable rings; and five of said at least threeretaining rings.
 15. The carrier assembly of claim 11, wherein each ofsaid at least three rings further comprises: an inner wall; an outerwall; and a plurality of transverse grooves from said inner wall to saidouter wall.
 16. The carrier assembly of claim 11, wherein said carrierprotrusion front face on each of said at least three carrier protrusionsis formed with a chamfer all the way around said carrier protrusionfront face.
 17. The carrier assembly of claim 16, wherein an acute anglebetween said chamfer and said carrier protrusion front face on each ofsaid at least three carrier protrusions is in a range from 0.02 degreesto 5 degrees.
 18. The carrier assembly of claim 11, wherein each of saidat least three carrier protrusions is formed with a step.
 19. Thecarrier assembly of claim 18, wherein each of said at least threecarrier protrusions further comprises an elastic ring positioned on saidstep.
 20. The carrier assembly of claim 18, wherein a height dimensionof said step formed on each of said at least three carrier protrusionsis selected to cause a selected magnitude of deflection of a part of awafer surface toward said base plate during wafer polishing.